This invention relates to integrated circuits (IC""s ), and more particularly to voltage-converter and bus relays without a direction control input.
Continued shrinking of physical device sizes in integrated circuit (IC) chips has necessitated the reduction of power-supply voltages. Smaller voltages are needed to prevent punch-through or other breakdown and failure of the smaller, more delicate transistors.
Power-supply voltages have been gradually lowered over the last several years, from 5.0-volts to 3.3, 2.5, 1.8, 1.5, and 1.2 volts. The result is that some IC chips operate with 3.3-volt signals, while others operate with 1.8-volt or other signal voltages. A designer of an electronic system may need to use IC chips with different signal-voltage levels. The system-level designer must carefully design interfaces between the different voltage domains. For example, a 1.8-volt microprocessor may need to be interfaced with bus-logic chips that use 2.5-volt signals.
Bus-interface chips may be placed between the different voltage domains.
These bus-interface chips can convert signals from one voltage to another. For example, a logic high signal of 2.5 volts can be stepped down to 1.8 volt, while logic lows of zero volts are passed through. In the reverse direction, a 1.8-volt input can be converted to a 2.5-volt output.
Often a direction control signal must be inputted to the bus-interface chip. FIG. 1 shows a bit slice of a bus-interface chip with a direction-control input. Bus-interface chip 10 converts signals between bus A and bus B.
An input signal such as direction (DIR) is generated by external bus-control logic. Input DIR is buffered by inverter 16 and enables forward buffer 14 and disables reverse buffer 12 when DIR is low, but enables reverse buffer 12 and disables forward buffer 14 when DIR is high. When DIR is high, external driver 26 drives a signal to bus B, which is buffered by reverse buffer 12 to drive bus A. Input buffer 22 reads this signal on bus A.
When DIR is low, external driver 20 drives a signal to bus A, which is buffered by forward buffer 14 to drive bus B. Input buffer 24 reads this signal on bus B.
Bus A and B may operate at the same high-level voltage, or may operate at different voltages. For example, the power-supply for bus A may be applied to reverse buffer 12, while the power-supply voltage for bus B is applied to forward buffer 14.
When the high-level bus voltages are the same, bus-interface chip 10 is considered to be a bus relay device. When the high-level voltages on the two busses are different, bus interface chip 10 is considered to be a voltage converter or voltage shifter.
While such a bus interface is useful, external generation of the direction control signal DIR may be difficult for some systems. Bus-handshake, timing, or strobe signals may need to be sampled from a microprocessor or other bus-master device to determine in what direction the signals are propagating and when.
Some systems and bus protocols may not have clear-cut signals that can be used to generate the direction signal. Some bus protocols may not have the direction signal while there is a need for connecting devices working at different voltages. There is also a need to use a buffer to extend the wiring trace for longer distance, or to use a buffer for higher speeds and for connecting more devices, while the direction signal is not available in some protocols. Communication relays may receive only data without timing or control signals. Timing may be unspecified when delays are large. Bus conflicts or contention may occur when two buffers drive the same bus line, such as can occur if the direction control input is in the wrong state, even for a short period of time. Damage may result to the devices.
What is desired is a bus-interface chip that does not have a direction-control input. A bi-directional bus-interface chip is desired that automatically determines direction without examining an externally-generated direction-control input. A voltage shifter without a direction-control input is desirable.